Switch-mode power-supply design can be
a mysterious thing because there is a great variety of topologies and controller
types from which to choose. This application brief series describes how to pick the
most fitting power supply topology for an application and the knowledge needed to
get there. The best starting point is usually a dedicated specification for the
application. This specification should at least include information about the
input-voltage range, output voltage, and maximum load current. However, choosing the
most fitting topology, system design, or both is easier if some of the subsequent
questions can be answered:
- Does the application need an
isolation barrier between the input and the output? If yes, which insulation
level is needed? Can the output-voltage regulation be achieved with primary- or
- Is the power supply intended for
DC-to-DC or for AC-to-DC conversion? Additional helpful information regarding
the input can include the maximum inrush current, the maximum input current, and
the maximum acceptable reflected ripple.
- What is the output power range
for the application? In many cases, this information decreases the number of
usable topologies and controllers. The specification should also include
requirements for the output-voltage tolerance of the power supply, maximum
acceptable output-voltage ripple, average output current, and peak output
current. Additional demands for dynamic behavior like load regulation, transient
response and line regulation (the latter is important for automotive cranking,
for example) must also be in the specifications, because the power stage
sometimes needs to be adjusted accordingly to meet the specifications.
- What is the desired switching
frequency? Is frequency dithering needed to lower peak emissions? Is there more
than one power supply in the system? If so, do the supplies need to be
synchronized? For automotive applications, choose a switching frequency below
450 kHz or above 2.1 MHz to avoid interference with the AM band. For high-power
applications, choose a low switching frequency for the best possible
- What is the ambient and working
temperature range? Which application segment is the design for? Are commercial,
automotive, military, or space-grade parts required?
- What is the main priority for the
power supply? In general, for every power-supply design, trade-offs are made
between performance, form factor, and cost. Know which of these factors has the
highest priority, because these factors directly impact the quality of the
- Does the power supply need to
meet certain standards regarding efficiency, electromagnetic interference (EMI),
power factor correction (PFC), or Underwriters Laboratories (UL) qualification?
Is light load efficiency or a specific standby power level required?
All of this information is not always
necessary. The more detailed the power-supply specification is, the easier it is to
pick the best-fitting topology and the best performing components.
The most common switch-mode
power-supply topologies are included in the following list:
primary-inductance converter (SEPIC)
- LLC Half-Bridge
- LLC Full-Bridge
topologies are supported by TI’s Power Stage Designer™ software tool.
Table 1-1 summarizes the most common parameters for power-supply specifications.
Table 1-1 Summary of Helpful
- DC/DC or
- Light load
Part 2 of this series describes how to pick the
most fitting topology based on the parameters of the specification.